Re: [Xen-devel] [PATCH] xen/balloon: flush unused mappings before updating P2M table

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 14/03/14 16:21, Wei Liu wrote:
> Xen balloon driver will update ballooned out pages' P2M entries to point
> to scratch page for PV guests. In 24f69373e2 ("xen/balloon: don't alloc
> page while non-preemptible", kmap_flush_unused was moved after the
> update for P2M table. In that case for 32 bit PV guest we might end up
> with
> 
> P2M    X -----> scratch_page
> M2P    Y -----> X  (Y is mfn in unused kmap entry)
> 
> When PVMMU is consulted, it gets confused and returns the wrong value.
> Eventually the guest crashes.
> 
> Move the flush before __set_phys_to_machine to fix this.

The scrub_page() will immediately repopulate the kmap cache with the MFN
about to be returned to Xen so this isn't the correct place.

I don't understand your description of the problem so I cannot suggest a
correct fix.  What's consulting what?

As an aside, I do think the flush_tlb_all() is unnecessary since Xen
does that for us in the update_va_mapping hypercall.  I think. Tim, can
you confirm?

David

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